Single mode optical fibers are expected to bring rapid advancements in a wide array of photonic applications, providing the high bandwidth that supports applications such as video links, fiber laser and fiber amplifier systems, and direct-write laser printing. The construction of single mode fibers eliminates intermodal dispersion and thereby affords extremely large bandwidths compared with multimode fibers. Tapping the potential of single mode fibers presents a significant challenge due to the extremely small aperture (8 .mu.m typically) making fiber-to-fiber and fiber-to-photonic device connections an exacting and expensive process. Attaining useful coupling efficiencies requires that the fiber be aligned with submicron accuracy and be fixed in place through the life of the device. This coupling process is even more demanding with lensed single mode fibers and Polarization-Maintaining (PM) fibers.
Significant challenges, both technical and economic, have to be surmounted in order for photonic devices using single mode fibers to become widely adopted, chief among them being the fiber alignment issue. Also, broader application of PM fibers in quantum well lasers, traveling wave amplifiers, switches, and other devices has been impeded by the alignment problem. For purposes of this application, the terminology "fiber optic", whether in its singular or plural form, will be understood to refer to single mode fibers.
For instance, the development of higher volume, lower cost opto-electronic manufacturing technologies has been desirable in order to accelerate the installation of opto-electronics in lower speed and shorter distance networks. These networks include important technologies such as telecommunications, and computer and video applications for both commercial and defense systems. Opto-electronics is a hybrid technology based on the integration of laser diodes, silicon detectors and integrated circuits, and GaAs high frequency circuits, and so forth, which currently are coupled to the external environment via silica fiber optics.
Proper alignment of the optical fibers is necessary in order to maximize the percentage of light coupled from the light source or electro-optic device to the optical fiber and to thereby increase the transmission efficiency of the optical signals. However, the alignment of optical fibers is complicated by the relatively small sizes of both the optical fiber waveguide, such as a single mode optical fiber which, for example, can have a light transmitting core diameter of approximately 2-10 micrometers, and the light source which has approximately the same emitting area size.
Furthermore, it is even more complicated to precisely align an optical fiber within a hermetically sealed package in which opto-electronic devices are typically disposed. As known in the art, in addition to precisely aligning the optical fiber in each of the six degrees of freedom, the alignment process must typically be performed without physically straining or otherwise heating the optical fiber and mount since heat can cause the optical fiber to move due to thermal expansion, thereby misaligning the optical fiber. Also, the heat required to allow one optical fiber to be positioned and affixed can oftentimes affect the position or alignment of adjacent optical fibers thereby misaligning the adjacent optical fibers and thus making multiple single mode fiber alignment and coupling extremely difficult. In addition, access to an optical fiber within a hermetically sealed package is generally limited since the optical device with which the optical fiber is being aligned is disposed within an internal cavity defined within the hermetic package, and typically drives the choice of a hermetic package that is larger than needed.
As a consequence of the above-mentioned difficulties in precisely aligning optical fibers, a need has persisted for a reliable, cost effective and easily implemented single mode fiberoptic alignment and bonding to discrete opto-electronic devices. Namely, the laser diode to fiber coupling has been the weakest link in the opto-electronic hybrid manufacturing process. As a consequence, the packaging of individual single mode opto-electronic components has been dominated by the fiber alignment issue, and which has represented in excess of 40-50% of the product cost. Additionally, the potential for development of revolutionary photonic technologies based on fiber optic interconnections has remained largely untapped because of the fiber alignment problem.
U.S. Pat. No. 5,602,955, the entire disclosure of which is incorporated herein by reference, describes various methods and apparatus known in the art for aligning an optical fiber as well as the shortcomings associated with those methods and apparatus. Contemporary procedures for aligning fiber optics and microptics have been very labor intensive and required considerable capital investment.
In general, prior optical fiber alignment approaches fall under either of two basic categories, viz., active and passive alignment techniques. Passive alignment relies on precision fixturing of the fiber relative to the coupled device. For instance, a known passive fiber alignment scheme employs a micromachined V-groove in a silicon substrate for precision placement of the fiber followed by bonding. The silicon micromachining permits very precise definition of the V-groove with respect to the fiber dimensions and to the silicon surface. Silicon V-grooves have been coupled with precision placement of solder bumps and surface tension alignment to provide one-dimensional fixturing and alignment of fibers.
Although the passive alignment approach is potentially more cost-effective than active alignment, prior systems have not yet achieved the manufacturable tolerances required by single mode fiberoptic alignment. Namely, prior published efforts on use of passive alignment for optical fiber alignments have reported laser diode to optical fiber transverse and lateral misalignments of approximately 2.8 microns for single mode coupling, which are not suitable for single mode packaging. E.g., see J. Sutherland et al., Optical Coupling and Alignment Tolerances in Optoelectronic Array Interface Assemblies, 1995 IEEE Electronic Components and Technology Conference, and Sutherland et al., Alignment Tolerance Measurement and Optical Coupling Modeling for Optoelectronic Array Interface Assemblies, 1996 IEEE Electronic Components and Technology Conference.
In active alignment, the powered (e.g., light-emitting) operator or machine manipulates the fiber relative to the coupled active device and seeks active feedback, such as the output from a photodiode, to optimize the alignment. Once optimized, the fiber is affixed in place using laser welding, adhesives or eutectic bonding. Commercial alignment systems have been developed to automatically connect, or pigtail, an opto-electronic device, such as a laser diode, to an optical fiber. For instance, the known Melles Griot and Newport alignment system has been developed to precisely align optical fibers with other optical components. These automated active alignment systems are very expensive to implement because they have the drawback of being package-specific and device-specific and, therefore, require retooling for each application. Such prior automated active alignment systems represent a large capital outlay that hindered product development at the startup, university, or experimenter level, or they were incompatible with the goal of providing versatile, high volume manufacturing due to the fact that it represents a choke point. Consequently, the task of fiber alignment is predominantly a manual operation at companies with low capitalization, in which an operator must work through a microscope with precision stages to manipulate and bond each fiber.
In an effort to address these issues, active fiber micro-aligners (AFMAs) have been developed to permit an optical fiber to be aligned with an electro-optic device, such as a laser diode or even another optical fiber. At the heart of AFMA technology are microelectromechanical systems (MEMS) devices, which are micromachined silicon chips, that possess both mechanical and electronic functionality.
U.S. Pat. No. 5,602,955 shows a microactuator which controllably positions an optical fiber without the need for heating during alignment thereby preventing misalignment of adjacent optical fibers. The microactuator of U.S. Pat. No. 5,602,955 is bimorphic in nature, and it precisely aligns an optical fiber within a hermetically sealed package, such as a standard 14-pin hermetic butterfly package, in which opto-electronic devices are typically disposed. While the microactuator of U.S. Pat. No. 5,602,955 provides a significant improvement over previous AFMA technologies, further improvements are nonetheless desired to reduce costs of opto-electronic hybrid manufacturing processing and to open-up opportunities in all-optical photonics applications.